Every time the heart squeezes it requires an electrical impulse. The impulse originates from an area in the top right corner of the heart known as the sinus node. Electricity then spreads across the upper two chambers of the heart. It travels to the lower two chambers through a specialized conducting pathway known as the atrioventricular (AV) node. Every piece of tissue in the heart is electrically active. All heart tissue can conduct electricity. However, in addition, all heart tissue can generate electricity as well. Normally the heart tissue is simply conducting electricity generated by the sinus node. However, occasionally it will generate it as well; when this happens a premature contraction results.
A PAC is a premature contraction coming from a piece of tissue in the atria. A PAC results in an electrical impulse coming earlier than expected, and in most cases causes an extra contraction of the heart. This in and of itself is not dangerous. In fact, almost everyone has at least a few PAC's every day! In this sense, PAC's can be considered a normal variant. Most people don't feel PAC's, although some people do seem to feel them. Occasionally a situation will arise when an individual is having PAC's fairly frequently. They may be happening so frequently that the doctor can appreciate them on a routine checkup. Usually when this occurs the individual is referred for further evaluation.
When PAC's are happening frequently, the question arises as to whether they are still a normal variant or not. Occasionally frequent PAC's may be a marker of some type of disease process in the heart. For example, PAC's may occur if the heart muscle is inflamed or irritated. They may also occur if there is some type of problem with the electrical conduction system. The job of the cardiologist is to determine whether a patient with frequent PAC's has any type of disease process that is causing them, or whether the PAC's are still normal but simply more frequent than most other people have.
Typically the evaluation of some someone with frequent PAC's involves an electrocardiogram (ECG), an echocardiogram to evaluate the appearance and function of the heart muscle, and a 24-hour recording of the heart rhythm (Holter monitor). The Holter monitor allows the doctor to assess the frequency and pattern of PAC's over a prolonged period of time. Provided the studies do not show any abnormalities other than isolated PAC's, usually it can be assumed that the PAC's are benign in nature, and simply more frequent than what is seen in most individuals. The prognosis for an individual with benign PAC's is generally excellent. No special medication treatment is needed; no special activity restrictions or precautions are needed either. It is often the case that the PAC's spontaneously decrease in frequency over time. Occasionally they may completely disappear. However, provided the PAC's are determined to be benign, there is no danger to the heart one way or the other.